Absolutely brilliant. It was slow to start, but I'm about a quarter of the way in now, and it's just incredible. There's an enormous variety of missions and things to do, a huge open world, two great protagonists (and then Franklin) and a whole host of great supporting characters. I'm going to flight school one minute, then taking photos of a sex scandal, then switching to Trevor to go on a shooting rampage.
A few niggles: storing vehicles is a bit of a pain; the pop-in can be pretty awful; Franklin is kind of dull and his friends are all intensely detestable; the checkpointing can be irritatingly rare at times. But these are all minor complaints.
It's not going to beat Infinite as GoTY for me, but it's competing with TLOU for second place. It's definitely in the top ten for this generation, and I've never really been a GTA fan.